The - tag should prevent trivial republishing; the rest is just good old pirating which will always exist.
Personally i think the zapbox relay model would work best:
follow = get free feed;
Pay = instantly just get the free+paid content aswell in the feed.
Most frictionless, client independent experience for the user. Publishers need to make sure they use the right relays that will comply with their chosen witholding scheme/metrics.
